Styling the "underline" color for the nav menu

Hi, I’m trying to change the color of the underline (don’t know what else to call it) on the nav menu in the header.
I tried this:
.avia-menu-fx {
background-color: #E8A3E5;
color: #ffffff;
border-color: #E8A3E5;
margin-bottom: 30px;
}
The margin worked but the colors are being overwritten and I can’t figure out what element is overwriting them. Any ideas?
Many thanks in advance.